This invention relates generally to the art of voice range audio communication, and more particularly such communication that takes place through a carbon granule microphone.
The nonlinear characteristics of carbon microphones are well known. Carbon granule microphones are utilized most widely in telephone handsets where the nonlinearity is of no particular concern for ordinary usage. But when such a microphone is used as part of a communication channel of digital information through a modem and acoustic coupler, the nonlinearities can be a problem. A modem typically operates to transmit and receive data within a voice range (that is, below substantially 3,000 Hz) over a telephone line. One type of modem does so by direct wire connection with the telephone line but another type of modem uses an acoustic coupler in which an ordinary telephone handset is placed. It is with the use of the acoustic coupler that the telephone carbon microphone nonlinear characteristics become part of the data transmission path.
A particular problem has recently appeared in this regard with a modem where in its receiving bandwidth is two times the transmission bandwidth. Therefore, any second harmonics generated from the transmitted signal will fall in the receive band and thus appear as noise that interferes with signals being received by the modem. When an acoustic coupler and certain types of carbon telephone microphones are used, these harmonics become strong enough that the data transmission errors are intolerable.
Therefore, it is an object of the present invention to provide a technique for reducing such harmonics.